The present invention relates to an exhaust gas recirculating type diesel engine wherein part of the exhaust gas emitted into the exhaust system of a diesel engine is recirculated into the intake system of the engine. More particularly, the present invention relates to an exhaust gas recirculating type diesel engine having an improved piston structure for reducing problems, such as mechanical abrasion and chemical corrosion of mechanical parts and deterioration of lubricants, caused by micro particle impurities containing sulphuric acid components and generated as products of combustion in the diesel engine.
In a conventional gasoline engine, fuel and air are first mixed with each other, and then the resulting combustible gas mixture is supplied to a combustion chamber. Micro particles are usually not generated after the gas mixture is burnt. Even when part of the exhaust gas is recirculated from the exhaust system to the intake system in order to reduce the amount of oxides of nitrogen (NO.sub.x) contained in the exhaust gas, no problems due to the generation of such small particles are encountered by such a gasoline engine.
Contrary to this, in a diesel engine, wherein fuel is injected into compressed air, small particles are generated when the engine is operating under a high load. However, in a conventional diesel engine, the maximum amount of fuel injection is limited so that the generation of small particles can be restricted to a level that does not prevent the diesel engine from being driven, although the output power of the diesel engine is rather lowered reduced from the maximum value. Recently, to comply with the requirements concerning the purification of exhaust gas, it has become necessary to apply exhaust gas recirculation to diesel engines to reduce emission of NO.sub.x. However, when exhaust gas is recirculated in a diesel engine, the combustion in its combustion chamber may be incomplete, and as a result, a large amount of small particles may often be generated in a driving region wherein exhaust gas is recirculated. Diesel fuel contains more sulphur components than gasoline, and accordingly, small particles generated through the above-mentioned exhaust gas recirculation also contain a large amount of sulphur components. When these small particles containing a large amount of sulphur components pass through the narrow space between the piston rings and the cylinder wall, they are admixed with lubricant, which is thereby deteriorated. In addition, when lubricant admixed with such small particles reaches the valve drive system of the diesel engine, both mechanical and chemical ill effects may occur in various parts of the diesel engine, such as abrasion and corrosion in the valve mechanism, pistons, piston rings, and cylinder liners, and chemical deterioration of the lubricant. These problems have been a large barrier against applying exhaust gas recirculation to a diesel engine.